1. Field of the Invention
The present invention is generally related to cores used in the casting process to provide cavities in a cast metallic object and, more particularly, to a composite core that comprises both soluble and insoluble portions.
2. Description of the Prior Art
Those skilled in the metal casting process, and particularly the high pressure die casting processes, are well aware that cavities in cast metal objects are often provided by using a soluble core that is inserted into a mold prior to causing a molten metal to flow into the mold. The soluble core, which is typically made of a salt compound, reserves space in the mold as the liquid metal flows around it and, after the molten metal has solidified, the soluble core can be dissolved to leave a cavity of the desired shape and size. The soluble cores are typically made of a salt compound and can be removed from the solidified metal casting by causing water or another liquid to flow in contact with the soluble core. This liquid dissolves the salt core, leaving a cavity having a size and internal surface configuration generally identical to the outer surface configuration of the salt core prior to the casting process.
U.S. Pat. No. 4,586,553, which issued to Allen et al on May 6, 1986, describes a process for pressure casting a piston with a crown insert and a cavity. The crown is placed in the mold before casting and the soluble salt core forms a cavity in the piston. The salt core is held by a crown insert to position the salt core in the mold to prevent the salt core from moving during the pressure casting procedure.
U.S. Pat. No. 5,803,151, which issued to Carden on Sep. 8, 1998, describes a soluble core method for manufacturing metal cast products. The improved soluble core for die casting metals or metal matrix composites is formed of a mixture of salt and up to about 20 weight % of ceramic material blended together to produce a homogeneous mixture and compacted under pressure to product a soluble core having little or no porosity. The ceramic material can be in the form of fibers, particulates, whiskers, and/or platelets, and has a melting temperature greater than that of the salt.
U.S. Pat. No. 3,963,818, which issued to Sakoda et al on Jun. 15, 1976, describes a water soluble core for pressure die casting and process for making the same. The process includes pre-drying a granular water soluble salt having grain size of less than about 1000 microns so that the moisture content thereof becomes less than 1%. It also comprises the step of molding under pressure the granular water soluble salt into a desired shape and volume at a pressure of between about 1.5 to 4 tons per square centimeter and, if necessary, sintering the molded salt at a temperature of between about 100xc2x0-300xc2x0 C. The core for pressure die casting acts as a cavity former within a casting and substantially consists of a water soluble salt having a compressive strength of between about 800-1480 kg per square centimeter, a bending strength of between about 200-370 kg per square centimeter, and a density of between 2.05-2.12.
U.S. Pat. No. 4,252,175, which issued to Whipple on Feb. 24, 1981, describes a cylinder block having a cast in core unit and a process for manufacturing the same. A core unit for use in casting a cylinder block of an internal combustion engine which core unit comprises a preformed cylinder liner which includes a cylindrical sidewall defining an interior bore and having a port in the sidewall is disclosed. A first core unit is formed of a reducible material molded upon the preformed liner, which first core unit includes a first main core portion which partially occupies the bore and a port core portion which extends through the port. A second core unit is formed of the reducible material separately from the first core unit and is assembled upon the cylinder liner, which second core unit includes a second main core portion in the bore and in mating alignment with the first main core portion, thereby forming a composite core assemblage.
U.S. Pat. No. 4,361,181, which issued to Wischnack et al on Nov. 30, 1982, describes a casting core and process for the production thereof. A casting core for the creation of difficultly accessible cavities in castings of aluminum or of one of its alloys, is produced from a water soluble salt as base substance and burnt sugar as a binding agent, and a process for the production of such a casting core wherein the base substance is mixed with burnt sugar in aqueous or organic solution, pressed into molds, and baked at elevated temperature.
U.S. Pat. No. 4,743,481, which issued to Quinlan et al on May 10, 1988, describes a molding process for articles having an irregular shaped internal passage. The process for making an article having an irregular internal passage utilizes a hollow polymer preform. The preform is filled with a relatively incompressible filler material such as a powder or a fluid, which supports the preform when it is placed in a mold, such as an injection mold. The filler enables the preform to withstand high molding pressures and prevents deflection and movement of the internal passage within the preform. The shell, a layer of a polymer material is then molded about the preform. After the final article has been formed, consisting of the preform and the shell, the filler is removed for possible reuse.
U.S. Pat. No. 4,840,219, which issued to Foreman on Jun. 20, 1989, describes a mixture and method for preparing casting cores and cores prepared thereby. Casting cores are fabricated from a mixture comprising a molten salt having dispersed therein a particulate material which includes a first refractory material having a mesh size of 60-120 and a second refractory material having a mesh size of at least 200. The salts are preferably halides, carbonates, sulfates, sulfites, nitrates or nitrites of Group Ia and Group IIa metals and the refractory material may be selected so as to be non-reactive with the molten salt. Some preferred refractory materials include alumina and magnesium silicate.
U.S. Pat. No. 4,922,863, which issued to Adams on May 8, 1990, describes a cast engine cylinder having an internal passageway and method of making same. A cast cylinder for an internal combustion engine having an intake valve cavity located on one side of the piston bore, an intake bore for communication with a carburetor located on the other side of the piston bore, and an internal passageway cast there in communicating the intake bore and the intake valve cavity is disclosed. The internal passageway is curved and circumscribes a portion of the intake bore. A walled hollow tube having initially closed ends is embedded in the cast cylinder during casting as a permanently retained casting core. Subsequently, the ends of the embedded tube are machined open to communicate with the intake valve cavity and the intake bore, respectively, to define the internal passageway.
U.S. Pat. No. 4,904,423, which issued to Foreman et al on Feb. 27, 1990, describes a pressure molding process using salt cores and composition for making cores. The process for pressure molding an article around a hardened salt mold core made from a mixture of relatively low melting temperature salt and sand, wherein the core is removed from the finished article by immersion of the article containing the core into a molten bath mixture of the core material to thereby melt the core out of the article, is disclosed. The process also recovers the core material and thus replenishes the bath for use in making additional cores. The bath is originally constituted by melting a suitable quantity of a dry premix of the salt and sand.
U.S. Pat. No. 5,303,761, which issued to Flessner et al on Apr. 19, 1994, describes a die casting process using casting salt cores. A process of providing a disposable core for use in die casting processes is described. A salt material is molten and cast into a core of a desired configuration under exacting conditions. The fluidity of the molten salt is controlled enabling casting the salt material into a core by die casting methods. The die casting method provides a core with a high surface finish and strength. The core is evenly cooled subsequent to it being cast and is maintained at an elevated temperature to maintain its surface finish and structural integrity. The cast core is inserted into the dies of a metal die casting machine to facilitate casting a metal product having internal forms not otherwise attainable. The core is removed from the metal product by simply dissolving and flushing the core out of the casting. The salt material may be reclaimed by a de-salination process for further use.
U.S. Pat. No. 4,875,517, which issued to Donahue et al on Oct. 24, 1989, discloses a method for producing salt cores for use in die casting. A pattern, identically proportional in configuration to the salt core to be produced, is initially formed from an evaporable foam material. The evaporable foam pattern is positioned in a mold and surrounded with an unbonded flowable material, such as sand. The pattern is contacted with a molten salt and the high temperature of the salt will vaporize the pattern, with the vapor being captured within the interstices of the sand while the molten salt will fill the void by vaporization of the foam to provide a salt core identical in configuration to the pattern. The salt core is subsequently used in a high pressure die casting operation to cast a metal part.
U.S. Pat. No. 5,165,464, which issued to Donahue et al on Nov. 24, 1992, discloses a method of casting hypereutectic aluminum-silicon alloys using a salt core. A method of high pressure casting of hypereutectic aluminum-silicon alloys using a salt core to form wear resistant articles, such as engine blocks is disclosed. To produce an engine block, one or more solid salt cores are positioned within a metal mold with the space between the cores and the mold defining a die cavity. A molten hypereutectic aluminum-silicon alloy containing more than 12% silicon is fed into the die cavity and on solidification of the molten alloy, precipitated silicon crystals are formed, which are distributed throughout the wall thickness of the cast part and also on the surface bordering the salt cores which constitute the cylinder bores in the cast block. The salt cores are subsequently removed from the cast block by contact with a solvent such as water.
The patents described above are hereby expressly incorporated by reference in the description of the present invention.
The use of soluble cores, such as those made of salt and other soluble materials, have been used in the casting industry for many years. Numerous ways are known to those skilled in the art for manufacturing soluble cores for these purposes. Molten salt can be injected into dies to form the salt cores. Lost foam casting processes can be used to form the salt cores. Other conventional casting processes, along with pressing technologies, are available to those skilled in the art for the manufacture of soluble cores that are used to provide cavities in finished cast metal objects.
Existing technology known to those skilled in the art for manufacturing and using salt cores has several inherent characteristics that can be improved. First, the salt often solidifies in a manner that forms cracks on the surface of the core. These cracks, during the injection molding process during which liquid metal is injected. into a cavity surrounding the core, can fill with metal. As a result, the metal that solidifies within the cracks of the salt core require a salvage operation for their removal. Secondly, large cross sectional areas of the expendable core significantly increase the cycle time during manufacture. This increased cycle time relates not only to the manufacture of the cores themselves, but also to the time required to dissolve the soluble material, such as salt, after the metal casting process is complete. Thirdly, positional issues arise when the expendable core is located in a steel casting die used to make a high pressure die casting, normally of aluminum. It is often difficult to precisely locate the salt core within the steel casting die prior to injecting liquid metal into the die during the high pressure die casting process. Finally, the core material has an associated raw material cost that is proportional to the core volume. This raw material cost, relating directly to the amount of salt used, is incorporated in the cost of the final cast metal object.
It would therefore be significantly beneficial to the metal casting process if a core could be provided which reduces the likelihood of cracking on the surface of the core, reduces the heavy cross sectional areas of the soluble material, allows the expendable core to be more accurately positioned within the steel casting die used during the high pressure die casting process, and reduce the total raw material cost of salt used in the process.
A composite core made in accordance with a preferred embodiment of the present invention for use in casting an object comprises a soluble portion of the composite core having an outer surface shaped to form an internal surface of a cavity in a metal object and a insoluble support member. The soluble portion is formed around at least a portion of the insoluble support member.
In a particularly preferred embodiment of the present invention, the insoluble support member is made of metal, such as aluminum, and a distal end of the insoluble support member extends out of the soluble portion to expose the distal end. This allows the distal end to be used for purposes of positioning the composite core within a steel casting die, or mold, prior to injection of molten metal. The insoluble support member comprises a base portion from which a metallic strut portion extends. The base portion is shaped to be held by the mold prior to the injection of molten metal.
Throughout the description of the present invention, the metallic strut will be alternatively referred to as a support strut, a locating strut, or a metallic strut. These terms are used because the strut is generally metallic in a most preferred embodiment, provides an important support function for the salt core, and has a distal end that serves an important locating function as will be described in greater detail below.
The present invention provides a method for casting an object which comprises the steps of providing an insoluble support member, forming a soluble portion around at least a portion of the insoluble support member, and providing a shaped outer surface of the soluble portion to be generally identical to a cavity in a metallic object to be cast. The method further comprises extending a distal end of the insoluble support member out of the soluble portion and disposing the insoluble support member, with the soluble portion formed around at least a portion of the insoluble support member, in a mold having an internal surface which is shaped to form the metallic object. The soluble portion is disposed at a location within the mold to create the cavity in the metallic object.
The method further comprises the steps of causing a liquid metal to flow into the mold and around the insoluble support member with the soluble portion formed around at least a portion of the insoluble support member, allowing the liquid metal to solidify, removing the metallic object from the mold with the soluble portion formed around at least a portion of the insoluble support member remaining in place within the solidified metallic object, dissolving the soluble portion, and removing the insoluble support member from the cavity of the metallic object. The distal end of the insoluble support member is disposed in contact with an internal surface of the mold to determine a position of the soluble portion within the mold.